Modulating Gas Furnace

ABSTRACT

A method controls a modulating gas furnace by monitoring a differential pressure associated with the modulating gas furnace, learning an intermediate value associated with an intermediate capacity between a minimum output capacity of the gas furnace and a maximum output capacity of the gas furnace, learning one of a high value associated with the maximum output capacity and a low value associated with a minimum output capacity, establishing an estimated operating curve using either the intermediate value and the low value or using the intermediate value and the high value, and operating the gas furnace in accordance with the estimated operating curve.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of the prior filed andco-pending U.S. patent application Ser. No. 12/611,616 filed on Nov. 3,2009, and entitled “Modulating Gas Furnace,” which is herebyincorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Heating, ventilation, and air conditioning systems (HVAC systems)sometimes incorporate gas furnaces for providing a heating effect totemperature controlled areas or comfort zones. Some gas furnacescomprise draft inducers that pull flue gases resulting from combustionthrough heat exchangers. It is known that draft inducers cannotdependably be factory set to a particular speed or flowrate in a mannerthat accommodates for the wide variation of installation furnaceconfigurations and transient pressure fluctuations that may be presentamongst different installation locations. For example, some gas furnacesmay be installed with substantially different lengths of pipingconnected to an exhaust vent. Accordingly, it is known to provide afurnace with a variable speed draft inducer, the speed or flowrate ofwhich may be adjusted once the gas furnace is installed and/or inoperation. Some gas furnaces provide systems configured to learnoperating speeds that are suitable for a particular installation of agas furnace. For example, U.S. Pat. No. 6,257,870 (referred tohereinafter as the '870 patent) and U.S. Pat. No. 5,791,332 disclosesystems and methods for operating a variable speed draft inducer of agas furnace to account for static and dynamic variations in heatexchanger pressure differential, H_(X)ΔP.

SUMMARY OF THE DISCLOSURE

In one embodiment, a method of controlling a modulating gas furnace isprovided that comprises monitoring a differential pressure associatedwith the modulating gas furnace and learning an intermediate valueassociated with an intermediate capacity between a minimum outputcapacity of the gas furnace and a maximum output capacity of the gasfurnace. The method further comprises learning one of a high valueassociated with the maximum output capacity and a low value associatedwith a minimum output capacity. The method further comprisesestablishing an estimated operating curve using either the intermediatevalue and the low value or using the intermediate value and the highvalue, and operating the gas furnace in accordance with the estimatedoperating curve.

In other embodiments, a gas furnace is provided that comprises avariable speed draft blower and a variable gas valve. The gas furnacefurther comprises a high pressure tap and a low pressure tap beingconfigured to monitor a pressure differential associated with the gasfurnace. The variable speed draft blower and the variable gas valve areconfigured for variations in operation in response to variations in thedifferential pressure. The variable speed draft blower speed isconfigured for variation in accordance with an estimated operatingcurve, the estimated operating curve being based on one of a maximumlearned value associated with a maximum output capacity of the gasfurnace and a minimum learned value associated with a minimum outputcapacity of the gas furnace. The estimated operating curve is also basedon an intermediate learned value associated with an intermediatecapacity between the minimum output capacity and the maximum outputcapacity.

In yet other embodiments, a modulating gas furnace is provided thatcomprises a variable speed draft blower, a modulating gas valve, a lowpressure tap and a high pressure tap configured to monitor a pressuredifferential of the gas furnace, a low pressure limit switch configuredto actuate in response to the pressure differential being substantiallyequal to a selected low pressure value, a high pressure limit switchconfigured to actuate in response to the pressure differential beingsubstantially equal to a selected high pressure value, and anintermediate pressure limit switch configured to actuate in response tothe pressure differential being substantially equal to a selectedintermediate pressure value, the intermediate pressure value beingbetween the low pressure value and the high pressure value. Variation inthe operation of the variable speed draft blower results in variation ofthe pressure differential and variation of the pressure differentialresults in variation of the modulating gas valve. The modulating gasfurnace is operated according to an estimated operating curve that isestablished either in response to the low pressure limit switch and theintermediate pressure limit switch being actuated or in response to theintermediate pressure limit switch and the high pressure limit switchbeing actuated.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and theadvantages thereof, reference is now made to the following briefdescription, taken in connection with the accompanying drawings anddetailed description, wherein like reference numerals represent likeparts.

FIG. 1 is a cut-away view of a modulating gas furnace according toembodiments of the disclosure;

FIG. 2 is a simplified block diagram of some control components of themodulating gas furnace of FIG. 1 according to embodiments of thedisclosure;

FIG. 3 is chart that illustrates two operating curves for the gasfurnace of FIG. 1;

FIG. 4 comprises a flow chart that illustrates a method of operating themodulating gas furnace of FIG. 1; and

FIG. 5 illustrates a general-purpose processor (e.g., electroniccontroller or computer) system suitable for implementing the severalembodiments of the present disclosure.

DETAILED DESCRIPTION

Some gas furnaces are configured as variable output capacity devices(also referred to as “modulating furnaces”). In some modulatingfurnaces, variable speed draft inducers may be controlled to operate atspeeds appropriate for various desired output capacities. However,during the process of determining the appropriate inducer speeds to beassociated with particular desired output capacities, current systemsmay undesirably overheat or underheat a temperature controlled area,possibly resulting in creating and uncomfortable temperature controlledspace. Accordingly, the present system provides, among other features, amodulating furnace that operates to determine the appropriate inducerspeeds while minimizing undesirable overheating and/or underheating of atemperature controlled space.

Further, the present disclosure provides modulating gas furnaces andmethods of operating modulating gas furnaces at desired outputcapacities in spite of the above-described static and dynamic factorsaffecting pressure differential. The present disclosure provides suchoperation without requiring operation of the furnace in a manner thatmay cause discomfort to persons in the space conditioned by the furnace.More specifically, the present disclosure provides systems and methodsfor operating a furnace in a manner that compensates for theabove-described static and dynamic factors affecting pressuredifferential by, in some cases, estimating an appropriate operatingcurve for use in controlling the draft blower until an actual operatingcurve is established. In some embodiments of the present disclosure, thegas furnace may learn two values, estimate a substantially linearoperating curve based on the two learned values, and operate the draftblower according to the estimated operating curve.

FIG. 1 shows a modulating gas furnace 10 that comprises substantialsimilarities to the gas furnace of U.S. Pat. No. 6,257,870 issued toGordon Jeffrey Hugghins et al. and which is hereby incorporated byreference in its entirety. However, the modulating gas furnace 10differs from the furnace of the '870 patent at least because the furnace10 comprises a modulating combustion system 14. It will be appreciatedthat the term, “modulating,” as used in this disclosure is meant toindicate that a system or device may be selectively operated atsubstantially any value over a range of performance values in a mannerconsistent with a control resolution of the system. Generally, thefurnace 10 is operable so that the furnace 10 may selectively perform atsubstantially any selected output capacity value (kBtu/Hr) ranging froma maximum output capacity (100% output capacity) to a minimum outputcapacity (e.g., in some embodiments, about 40% of the maximum outputcapacity) with the modulating combustion system 14 capable of beingconstantly operated over a range of output capacities.

The modulating combustion system 14 is housed within the cabinet 12 andcomprises a burner assembly 16, a modulating gas valve assembly 18, anda control assembly 20. The furnace 10 further comprises a heat exchangerassembly 22 which comprises a plurality of heat exchangers 24, avariable speed induced draft blower 26, and a variable speed circulatingair blower 28. It will be appreciated that the furnace 10 furthercomprises a combustion intake space 30 that surrounds the exterior ofthe draft blower 26 and the exterior of the heat exchangers 24. When thedraft blower 26 draft is operated, air is drawn from the intake space 30and is passed through the heat exchangers 24 and into a header 34 thataccepts exhaust from the heat exchangers 24 and provides a flow path forthe exhaust to reach the draft blower 26. It will be appreciated thatduring operation of the furnace 10, the local pressure within the intakespace 30 may be different from the local pressure within the header 34.

The pressure difference that exists between the intake space 30 and theheader 34 is referred to as the combustion system pressure differential,or alternatively, may simply be referred to as the heat exchangerpressure differential (H_(X)ΔP) or simply pressure differential. It isfurther understood by those of ordinary skill in the art of gas furnacesthat the pressure differential may depend or vary in response to thephysical nature of an exhaust vent 32 connected downstream of the draftblower 26, atmospheric conditions that affect the pressure within theintake space 30 and the header 34, and the speed of operation of thedraft blower 26, among other factors. For example, the exhaust vent 32and any other structure joined downstream of the exhaust vent 32 mayexperience a buildup of condensation within the interior of the exhaustvent 32 and attached devices. Such a buildup of condensation mayincrease resistance to fluid flow through the exhaust vent 32 which mayincrease the above-described pressure differential. Similarly, if theexhaust vent 32 is vented to an exterior of a building that is exposedto variations in wind speed or external barometric pressure, a change inwind speed or external barometric pressure may also cause variation inthe pressure differential. Of course, changes in pressure local to theintake space 30 also may cause variation in the pressure differential.

FIG. 2 shows an embodiment of the control assembly 20 as connected tovarious system components, including the draft blower 26. In theembodiment of FIG. 2, the draft blower 26 comprises a motor 36 fordriving a shaft 38 which drives a blower wheel or fan 40. The motor 36is a variable speed motor capable of sensing an operating speed and anoperating torque of the motor 36 and communicating the operating speedand operating torque values to the control assembly 20. In thisembodiment, the control assembly 20 is connected to the motor 36 by acommunications transmit line 42 and a communications receive line 44. Ofcourse, in other embodiments, the above-described bidirectionalcommunication capability between the control assembly 20 and the motor36 may be accomplished in any other suitable manner. Further, in someembodiments, communication between the control assembly 20 and the motor36 may comprise use of digital serial communication methods. The controlassembly 20 is connected to the modulating gas valve assembly 18 bycontrol line 50. A flame sensor 52 and an igniter 54 are connected tothe control assembly 20 by electrical lines 58 and 60, respectively.

Referring now to both FIGS. 1 and 2, the furnace 10 further comprisesthree pressure switches, a low pressure limit switch 64, an intermediatepressure limit switch 66, and a high pressure limit switch 68. Each ofthe pressure switches 64, 66, and 68 may be implemented as switcheswhich open below desired pressure limits and close above the desiredpressure limits. However, in alternative embodiments, the pressureswitches 64, 66, and 68 may be replaced by pressure sensors suitable forsending analog or digital signals to control assembly 20. In thisembodiment, the pressure switches 64, 66, and 68 are connected to thecontrol assembly by pressure signal lines 70, 72, and 74, respectively.Each of the switches 64, 66, and 68 measure the pressure differentialthrough the use of an upstream pressure tap 76 configured to monitor thepressure of the combustion intake space 30 and a downstream pressure tap78 configured to monitor the pressure within the header 34. Inalternative embodiments, the pressure taps 76 and 78 may be placed tomonitor pressure of other locations that similarly provide pressurefeedback necessary to operate switches 64, 66, and 68 in response to thepressure differential. It will further be appreciated that upstreampressure tap 76 and downstream pressure tap 78 are also pneumaticallyconnected to modulating gas valve assembly 18 so that variations in thepressure differential result in substantially proportional variations infuel gas provided to the burner assembly 16 by the modulating gas valveassembly 18.

Accordingly, the furnace 10 may be controlled to provide a desiredoutput capacity by first controlling the speed of the induced draftblower 26, which affects the pressure differential and may cause themodulating gas valve assembly 18 to modulate to provide an appropriategas fuel flow in response to the sensed pressure differential.Generally, this operation is possible due to the predicable andsubstantially proportional relationships between changes in draft blower26 speed or RPM and the resultant changes in pressure differential andoxygen provided to the burner assembly 16 for combustion. In operation,changes in the induced draft blower 26 speed cause proportional andappropriate changes in the fuel gas provided by the modulating gas valveassembly 18.

The draft inducer motor 36 further comprises an integral controller 80configured to communicate with the control assembly 20 regarding thestatus of the switches 64, 66, and 68. In alternative embodiments, thestatus of the switches 64, 66, and 68 may be input directly to theintegral controller 80 via pressure signal lines 70, 72, and 74,respectively. In this disclosure, references to the draft blower motor36 also refer to the component parts of the motor 36, including theintegral controller 80. The motor 36 and/or the control assembly 20 maycomprise control algorithms suitable for determining suitable operatingspeeds for the draft blower 26.

Referring now to FIG. 3, two actual operating curves of the modulatinggas furnace 10 are shown. A lower actual operating curve 200 is shown asa substantially linear curve extending from about 40% output capacity to100% output capacity. The lower actual operating curve 200 isrepresentative of the draft blower 26 speed needed to cause themodulating gas valve assembly 18 and other components of the furnace 10to operate at specified output capacities. In this embodiment, a lowoperating point 202 is associated with the draft blower 26 speedrequired to provide a low output capacity. In some embodiments, the lowoutput capacity may have a value of 40% output capacity. Intermediateoperating point 204 is associated with the draft blower 26 speedrequired to provide an intermediate output capacity. In someembodiments, the intermediate output capacity may have a value of 65%output capacity. High operating point 206 is associated with the draftblower 26 speed required to provide a high output capacity. In someembodiments, the high output capacity may have a value of 100% operatingcapacity.

The actual operating curve 200 is appropriate for use in controlling thefurnace 10 under a first set of pressure conditions that yield a firstpressure differential. However, if the pressure conditions change to asecond set of pressure conditions yielding a pressure differential valuehigher than the first pressure differential, the actual operating curve208 may become the appropriate curve to use in controlling the furnace10. It will be appreciated that under the second set of pressureconditions, the draft blower 26 speed associated with low, intermediate,and high operating points 210, 212, 214, although higher in speed valuesthan points 202, 204, 206, respectively, are required to provide thesame low, intermediate, and high output capacities.

Further, it can be seen that while the differential pressures P_(L),P_(I), and P_(H) required to operate the furnace 10 at low,intermediate, and high output capacities, respectively, remain constantregardless of changes in pressure conditions. Such constantrelationships between differential pressure and output capacity allowslow pressure limit switch 64 (when configured to actuate at P_(L)),intermediate pressure limit switch 66 (when configured to actuate atP_(I)), and high pressure limit switch 68 (when configured to actuate atP_(H)) to provide information to motor 36 and/or control assembly 20.Such information may be used by the draft blower 26 and/or controlassembly 20 to capture and/or store appropriate draft blower 26 speedvalues at which the draft blower 26 must be operated to result in thefurnace 10 operating at the respective output capacities. It will beappreciated that a furnace 10 may need to determine an appropriateoperating curve in order for the furnace 10 to reliably be operated at aselected output capacity. In some embodiments, such determination may beaccomplished by learning values for variables, LOW, INTERMEDIATE, and/orHIGH (each described in greater detail below).

Referring now to FIGS. 4, a method 300 of controlling the modulating gasfurnace 10 is shown (FIG. 4). A furnace 10 may be operated according tothe method so that undesirable overheating and/or underheating of acontrolled space may be reduced by establishing an estimated operatingcurve based on only two operating points and thereafter varying theoutput capacity and associated draft blower 26 speed in accordance withthe estimated operating curve. For purposes of this discussion, it willbe understood that the furnace 10 may comprise a thermostat and/or otherdevices configured to determine and communicate to the control assembly20 a DEMAND value (in some embodiments, expressed in terms of outputcapacity percentage) that represents the current system requirementand/or request for heat. Further, it will be appreciated that thevariables, LOW, INTERMEDIATE, and HIGH, may be used to store variousdraft blower 26 speeds (RPM) that generate the differential pressures,P_(L), P_(I), and P_(H), respectively. Still further, the variables,ESTIMATED OPERATING CURVE and ACTUAL OPERATING CURVE, may beconceptualized as simplified representations of the data sets ofinformation required to calculate substantially linear estimatedoperating curves (based on either LOW and INTERMEDIATE values or basedon INTERMEDIATE and HIGH values) and actual operating curves (based onLOW and HIGH values).

The method 300 may start at block 302, for example, in response to thefurnace 10 first being run in a heating mode and before any of theabove-described variables have been learned.

Proceeding to block 304, if the DEMAND value is equal to or greater thanthe low output capacity and the DEMAND value is less than or equal tothe intermediate output capacity, the method proceeds to block 306.However, if the DEMAND value is not equal to or greater than the lowoutput capacity and the DEMAND value is less than or equal to theintermediate output capacity, the method proceeds to block 324.

At block 306, the method determines whether HIGH and INTERMEDIATE havebeen learned. From block 306, the method proceeds to block 308 if HIGHand INTERMEDIATE have not been learned. However, from block 306, themethod proceeds to block 342 if HIGH and INTERMEDIATE have been learned.

At block 308, the method determines whether LOW has been learned. Fromblock 308, the method proceeds to block 310 if LOW has not been learned.However, from block 308, the method proceeds to block 316 if LOW hasbeen learned.

At block 310, the method learns INTERMEDIATE. In some embodiments, thefurnace 10 may be operated to incrementally approach the intermediateoutput capacity by incrementally changing the draft blower 26 speed. Insome embodiments, the intermediate pressure limit switch 66 may beactuated from a closed circuit position to an open circuit position inresponse to the furnace 10 operating at the intermediate outputcapacity. Alternatively, in other embodiments, the intermediate pressurelimit switch 66 may be actuated from an open circuit position to aclosed circuit position in response to the furnace 10 operating at theintermediate output capacity. Still further, in other embodiments, apressure sensor or other suitable device may detect and/or communicate asignal that indicates the furnace 10 is operating at the intermediateoutput capacity. Regardless of the manner in which the components of thefurnace 10 accomplish such, at block 310, the value of INTERMEDIATE isset to the draft blower 26 speed at which the draft blower 26 was beingoperated when the furnace 10 achieved operation at the intermediateoutput capacity. Accordingly, it will be appreciated that operating thedraft blower 26 at the INTERMEDIATE value will thereafter result inoperation of the furnace 10 at the intermediate output capacity as longas static and dynamic variations in pressure differential aresubstantially the same as when the INTERMEDIATE value was stored. Afterlearning the INTERMEDIATE value, the method proceeds to block 312.

At block 312, the method learns LOW. In some embodiments, the furnace 10may be operated to incrementally approach the low output capacity byincrementally changing the draft blower 26 speed. In some embodiments,the low pressure limit switch 64 may be actuated from a closed circuitposition to an open circuit position in response to the furnace 10operating at the low output capacity. Alternatively, in otherembodiments, the low pressure limit switch 64 may be actuated from anopen circuit position to a closed circuit position in response to thefurnace 10 operating at the low output capacity. Still further, in otherembodiments, a pressure sensor or other suitable device may detectand/or communicate a signal that indicates the furnace 10 is operatingat the low output capacity. Regardless of the manner in which thecomponents of the furnace 10 accomplish such, at block 312, the value ofLOW is set to the draft blower 26 speed at which the draft blower 26 wasbeing operated when the furnace 10 achieved operation at the low outputcapacity. Accordingly, it will be appreciated that operating the draftblower 26 at the LOW value will thereafter result in operation of thefurnace 10 at the low output capacity as long as static and dynamicvariations in pressure differential are substantially the same as whenthe LOW value was stored. After learning the LOW value, the methodproceeds to block 314.

At block 314, the method establishes and ESTIMATED OPERATING CURVE. Insome embodiments, the ESTIMATED OPERATING CURVE comprises the necessarydata and/or information to establish a substantially linear curvegenerally fit to the values of LOW and INTERMEDIATE. Further,establishing the ESTIMATED OPERATING CURVE may comprise establishing atable of draft blower 26 speed values at which the draft blower 26 mustbe operated to achieve selected output capacity values ranging from thelow output capacity to the intermediate output capacity. It will beappreciated that any number of mathematical methods of establishing sucha table may be implemented to result in a set of values that form asubstantially linear curve. After the ESTIMATED OPERATING CURVE has beenestablished, in some embodiments including writing and/or storing theset of values in a memory device in a retrievable manner, the methodproceeds to block 316.

At block 316, the method operates the furnace 10 according to theESTIMATED OPERATING CURVE. In some embodiments, the method may look upand/or retrieve from a table containing values of the ESTIMATEDOPERATING CURVE a draft blower 26 speed required to operate the furnace10 at the output capacity value of DEMAND. After retrieving anappropriate draft blower 26 speed value, the method may force operationof the draft blower 26 at the retrieved value. From block 316, themethod proceeds to block 318.

At block 318, the method determines whether the DEMAND value has beenchanged. If the DEMAND value has not changed, the method proceeds backto block 316 and continues operation as described above. However, if theDEMAND value has changed, the method proceeds to block 320.

At block 320, the method determines whether the furnace 10 has requesteda RELEARN. In this embodiment, a RELEARN may be a command generated bythe furnace 10 that requires relearning of LOW, INTERMEDIATE, HIGH,ACTUAL OPERATING CURVE, and ESTIMATED OPERATING CURVE values. In thisembodiment, the furnace 10 may be configured to issue a RELEARN commandin response to substantial and/or sustained variation in the staticand/or dynamic factors (other than the draft blower 26 operating speed)which contribute to the pressure differential. Relearning is appropriatein such cases because operating according to the already determinedoperating curves may no longer result in operating the furnace 10 at theoutput capacity requested by the DEMAND value. If the method determinesthat a RELEARN has not been requested, the method proceeds to block 304.However, if the method determines that a RELEARN has been requested, themethod proceeds to block 322.

At block 322, the LOW, INTERMEDIATE, HIGH, ACTUAL OPERATING CURVE, andESTIMATED OPERATING CURVE values are discarded and/or removed frommemory. Next, the method proceeds to block 304.

Referring again to block 306, if at block 306 the method determines thatHIGH and INTERMEDIATE have been learned (i.e., currently have values),the method proceeds to block 342.

At block 342, LOW is learned. The operation of block 342 issubstantially the same as the operation of block 312. After LOW islearned, the method proceeds to block 344.

At block 344, the method establishes an ACTUAL OPERATING CURVE. In someembodiments, the ACTUAL OPERATING CURVE comprises the necessary dataand/or information to establish a substantially linear curve generallyfit to the values of LOW and HIGH. Further, establishing the ACTUALOPERATING CURVE may comprise establishing a table of draft blower 26speed values at which the draft blower 26 must be operated to achieveselected output capacity values ranging from the low output capacity tothe high output capacity. It will be appreciated that any number ofmathematical methods of establishing such a table may be implemented toresult in a set of values that form a substantially linear curve. Afterthe ACTUAL OPERATING CURVE has been established, in some embodimentsincluding writing and/or storing the set of values in a memory device ina retrievable manner, the method proceeds to block 346.

At block 346, the method operates the furnace 10 according to the ACTUALOPERATING CURVE. In some embodiments, the method may look up and/orretrieve from a table containing values of the ACTUAL OPERATING CURVE adraft blower 26 speed required to operate the furnace 10 at the outputcapacity value of DEMAND. After retrieving an appropriate draft blower26 speed value, the method may force operation of the draft blower 26 atthe retrieved value. From block 346, the method proceeds to block 350.

At block 350, the method determines whether the furnace 10 has requesteda RELEARN. The operation of block 350 is substantially similar to theoperation of block 320. If the method determines that a RELEARN has notbeen requested, the method proceeds back to block 346. However, if themethod determines that a RELEARN has been requested, the method proceedsto block 322.

Referring again to block 304, if at block 304 the method determines thatDEMAND is not greater than or equal to the low output capacity and lessthan or equal to the intermediate output capacity, the method proceedsto block 324.

At block 324, the method determines whether DEMAND is greater than theintermediate output capacity. If the value of DEMAND is not greater thanthe intermediate output capacity, the method proceeds back to block 304.However, if the method determines that DEMAND is greater than theintermediate output capacity, the method proceeds to block 326.

At block 326, the method determines whether HIGH has been learned. Ifthe method determines that HIGH has not been learned the method proceedsto block 328. However, if the method determines that HIGH has beenlearned, the method proceeds to block 330.

At block 328, the method learns HIGH. In some embodiments, the furnace10 may be operated to incrementally approach the high output capacity byincrementally changing the draft blower 26 speed. In some embodiments,the high pressure limit switch 68 may be actuated from a closed circuitposition to an open circuit position in response to the furnace 10operating at the high output capacity. Alternatively, in otherembodiments, the high pressure limit switch 68 may be actuated from anopen circuit position to a closed circuit position in response to thefurnace 10 operating at the high output capacity. Still further, inother embodiments, a pressure sensor or other suitable device may detectand/or communicate a signal that indicates the furnace 10 is operatingat the high output capacity. Regardless of the manner in which thecomponents of the furnace 10 accomplish such, at block 328, the value ofHIGH is set to the draft blower 26 speed at which the draft blower 26was being operated when the furnace 10 achieved operation at the highoutput capacity. Accordingly, it will be appreciated that operating thedraft blower 26 at the HIGH value will thereafter result in operation ofthe furnace 10 at the high output capacity as long as static and dynamicvariations in pressure differential are substantially the same as whenthe HIGH value was stored. After learning the HIGH value, the methodproceeds to block 330.

At block 330, the method determines whether LOW has been learned. If themethod determines that LOW has not been learned, the method proceeds toblock 332. However, if the method determines that LOW has been learned,the method proceeds to block 352.

At block 332, INTERMEDIATE is learned. Operation at block 332 issubstantially similar to operation at block 310. After INTERMEDIATE islearned, the method proceeds to block 334.

At block 334, the method establishes an ESTIMATED OPERATING CURVE.Operation at block 334 is substantially similar to operation of block314 except that the ESTIMATED OPERATING CURVE generated at block 334 isbased on the values, INTERMEDIATE and HIGH, rather than LOW andINTERMEDIATE. Accordingly, the ESTIMATED OPERATING CURVE generated atblock 334 may provide a table of values useful in operating the furnace10 at output capacities ranging from about the intermediate capacityoutput to the high output capacity. After the ESTIMATED OPERATING CURVEhas been established, in some embodiments including writing and/orstoring the set of values in a memory device in a retrievable manner,the method proceeds to block 336.

At block 336, the method operates the furnace 10 according to theESTIMATED OPERATING CURVE generated at block 334. In some embodiments,the method may look up and/or retrieve from a table containing values ofthe ESTIMATED OPERATING CURVE a draft blower 26 speed required tooperate the furnace 10 at the output capacity value of DEMAND. Afterretrieving an appropriate draft blower 26 speed value, the method mayforce operation of the draft blower 26 at the retrieved value. Fromblock 336, the method proceeds to block 338.

At block 338, the method determines whether the DEMAND value has beenchanged. If the DEMAND value has not changed, the method proceeds backto block 336 and continues operation as described above. However, if theDEMAND value has changed, the method proceeds to block 340.

At block 340, the method determines whether the furnace 10 has requesteda RELEARN. The operation of block 340 is substantially similar to theoperation of block 320. If the method determines that a RELEARN has notbeen requested, the method proceeds back to block 304. However, if themethod determines that a RELEARN has been requested, the method proceedsto block 322.

Referring back to block 330, if the method determines that LOW has beenlearned, the method proceeds to block 352.

At block 352, the method establishes an ACTUAL OPERATING CURVE.Operation at block 352 is substantially similar to the operation ofblock 344. After the ACTUAL OPERATING CURVE has been established, insome embodiments including writing and/or storing the set of values in amemory device in a retrievable manner, the method proceeds to block 354.

At block 354, the method operates the furnace 10 according to the ACTUALOPERATING CURVE generated at block 352. The operation at block 354 issubstantially similar to the operation at block 346. From block 354, themethod proceeds to block 356.

At block 356, the method determines whether the furnace 10 has requesteda RELEARN. The operation of block 356 is substantially similar to theoperation of block 320. If the method determines that a RELEARN has notbeen requested, the method proceeds back to block 354. However, if themethod determines that a RELEARN has been requested, the method proceedsto block 322.

It will be appreciated that when the furnace 10 is operated according tothe method 300, the furnace 10 may operate to satisfy a variety ofDEMAND values from the low output capacity to the intermediate outputcapacity in accordance with an ESTIMATED OPERATING CURVE without firsthaving to operate the furnace 10 at output capacities substantiallygreater than the intermediate output capacity. Similarly, when thefurnace 10 is operated according to the method 300, the furnace 10 mayoperate to satisfy a variety of DEMAND values from about theintermediate output capacity to the high output capacity in accordancewith an ESTIMATED OPERATING CURVE without first having to operate thefurnace 10 at output capacities of about the intermediate outputcapacity or lower. Still further, it will be appreciated that the method300 provides for operation of the furnace 10 according to an ACTUALOPERATING CURVE after having first operated the furnace 10 according toan ESTIMATED OPERATING CURVE. In some embodiments, the ACTUAL OPERATINGCURVE may more accurately provide operation speed values for the draftblower 26 than an ESTIMATED OPERATING CURVE according to which thefurnace 10 was operated prior to determining the ACTUAL OPERATING CURVE.

In alternative embodiments, the output capacity percentages associatedwith each of LOW, INTERMEDIATE, and HIGH may be set at values other than40%, 65%, and 100%. However in some embodiments, the output capacityassociated with LOW, the pressure P_(L), and the low pressure limitswitch 64 (when configured to actuate at P_(L)) may be set as any othervalue below which may be undesirable to operate the modulatingcombustion system 14 because of a high risk of flame extinguishment.Similarly, the output capacity associated with HIGH, the pressure P_(H),and high pressure limit switch 68 (when configured to actuate at P_(H))may be set at any other output capacity above which value furnace 10 isnot required to operate. Further, the output capacity associated withINTERMEDIATE, the pressure P_(I), and intermediate pressure limit switch66 (when configured to actuate at P_(I)) may be set at any other valuebetween the output capacities associated with LOW and HIGH.

Referring now to FIG. 5, the furnace 10 or associated components maycomprise a processing component (as a component of draft blower 26and/or control assembly 20) that is capable of executing instructionsrelated to the actions described previously. The processing componentmay be a component of a computer system. FIG. 5 illustrates a typical,general-purpose processor (e.g., electronic controller or computer)system 1300 that includes a processing component 1310 suitable forimplementing one or more embodiments disclosed herein. In addition tothe processor 1310 (which may be referred to as a central processor unitor CPU), the system 1300 might include network connectivity devices1320, random access memory (RAM) 1330, read only memory (ROM) 1340,secondary storage 1350, and input/output (I/O) devices 1360. In somecases, some of these components may not be present or may be combined invarious combinations with one another or with other components notshown. These components might be located in a single physical entity orin more than one physical entity. Any actions described herein as beingtaken by the processor 1310 might be taken by the processor 1310 aloneor by the processor 1310 in conjunction with one or more componentsshown or not shown in the drawing.

The processor 1310 executes instructions, codes, computer programs, orscripts that it might access from the network connectivity devices 1320,RAM 1330, ROM 1340, or secondary storage 1350 (which might includevarious disk-based systems such as hard disk, floppy disk, optical disk,or other drive). While only one processor 1310 is shown, multipleprocessors may be present. Thus, while instructions may be discussed asbeing executed by a processor, the instructions may be executedsimultaneously, serially, or otherwise by one or multiple processors.The processor 1310 may be implemented as one or more CPU chips.

The network connectivity devices 1320 may take the form of modems, modembanks, Ethernet devices, universal serial bus (USB) interface devices,serial interfaces, token ring devices, fiber distributed data interface(FDDI) devices, wireless local area network (WLAN) devices, radiotransceiver devices such as code division multiple access (CDMA)devices, global system for mobile communications (GSM) radio transceiverdevices, worldwide interoperability for microwave access (WiMAX)devices, and/or other well-known devices for connecting to networks.These network connectivity devices 1320 may enable the processor 1310 tocommunicate with the Internet or one or more telecommunications networksor other networks from which the processor 1310 might receiveinformation or to which the processor 1310 might output information.

The network connectivity devices 1320 might also include one or moretransceiver components 1325 capable of transmitting and/or receivingdata wirelessly in the form of electromagnetic waves, such as radiofrequency signals or microwave frequency signals. Alternatively, thedata may propagate in or on the surface of electrical conductors, incoaxial cables, in waveguides, in optical media such as optical fiber,or in other media. The transceiver component 1325 might include separatereceiving and transmitting units or a single transceiver. Informationtransmitted or received by the transceiver 1325 may include data thathas been processed by the processor 1310 or instructions that are to beexecuted by processor 1310. Such information may be received from andoutput to a network in the form, for example, of a computer databaseband signal or signal embodied in a carrier wave. The data may beordered according to different sequences as may be desirable for eitherprocessing or generating the data or transmitting or receiving the data.The baseband signal, the signal embedded in the carrier wave, or othertypes of signals currently used or hereafter developed may be referredto as the transmission medium and may be generated according to severalmethods well known to one skilled in the art.

The RAM 1330 might be used to store volatile data and perhaps to storeinstructions that are executed by the processor 1310. The ROM 1340 is anon-volatile memory device that typically has a smaller memory capacitythan the memory capacity of the secondary storage 1350. ROM 1340 mightbe used to store instructions and perhaps data that are read duringexecution of the instructions. Access to both RAM 1330 and ROM 1340 istypically faster than to secondary storage 1350. The secondary storage1350 is typically comprised of one or more disk drives or tape drivesand might be used for non-volatile storage of data or as an over-flowdata storage device if RAM 1330 is not large enough to hold all workingdata. Secondary storage 1350 may be used to store programs orinstructions that are loaded into RAM 1330 when such programs areselected for execution or information is needed.

The I/O devices 1360 may include liquid crystal displays (LCDs), touchscreen displays, keyboards, keypads, switches, dials, mice, track balls,voice recognizers, card readers, paper tape readers, printers, videomonitors, transducers, sensors, or other well-known input or outputdevices (i.e., a thermostat). Also, the transceiver 1325 might beconsidered to be a component of the I/O devices 1360 instead of or inaddition to being a component of the network connectivity devices 1320.Some or all of the I/O devices 1360 may be substantially similar tovarious components depicted in the previously described FIGS. 1 and 2.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations should be understood to include iterative rangesor limitations of like magnitude falling within the expressly statedranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4,etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit, RI, and an upper limit,Ru, is disclosed, any number falling within the range is specificallydisclosed. In particular, the following numbers within the range arespecifically disclosed: R=RI+k*(Ru−RI), wherein k is a variable rangingfrom 1 percent to 100 percent with a 1 percent increment, i.e., k is 1percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . 50 percent,51 percent, 52 percent, . . . , 95 percent, 96 percent, 97 percent, 98percent, 99 percent, or 100 percent. Moreover, any numerical rangedefined by two R numbers as defined in the above is also specificallydisclosed. Use of the term “optionally” with respect to any element of aclaim means that the element is required, or alternatively, the elementis not required, both alternatives being within the scope of the claim.Use of broader terms such as comprises, includes, and having should beunderstood to provide support for narrower terms such as consisting of,consisting essentially of, and comprised substantially of. Accordingly,the scope of protection is not limited by the description set out abovebut is defined by the claims that follow, that scope including allequivalents of the subject matter of the claims. Each and every claim isincorporated as further disclosure into the specification and the claimsare embodiment(s) of the present invention.

1. A modulating gas furnace, comprising: at least one general-purposeprocessor system configured to: monitor a differential pressureassociated with the modulating gas furnace; learn an intermediate valueassociated with an intermediate capacity between a minimum outputcapacity of the gas furnace and a maximum output capacity of the gasfurnace; learn one of a high value associated with the maximum outputcapacity and a low value associated with a minimum output capacity;establish an estimated operating curve using either the intermediatevalue and the low value or using the intermediate value and the highvalue; and control the gas furnace in accordance with the estimatedoperating curve.
 2. The modulating gas furnace of claim 1, wherein theestimated operating curve is established using the intermediate valueand the low value if a demand for heat is equal to or greater than theminimum capacity and is less than or equal to the intermediate capacityand wherein the estimated operating curve is established using theintermediate value and the high value if the demand for heat is greaterthan the intermediate capacity.
 3. The modulating gas furnace of claim1, wherein the at least one general-purpose processor system is furtherconfigured to: learn the high value if the estimated operating curve isbased on the intermediate value and the low value; learn the low valueif the estimated operating curve is based on the intermediate value andthe high value; establish an actual operating curve based on the lowvalue and the high value; and operate the modulating gas furnace inaccordance with the actual operating curve.
 4. The modulating gasfurnace of claim 3, wherein at least one of the low value and the highvalue and at least one of the estimated operating curve and the actualoperating curve are relearned in response to a relearn command.
 5. Themodulating gas furnace of claim 1, wherein the minimum capacity is setto a value below which a gas flame of the gas furnace is likely toextinguish.
 6. The modulating gas furnace of claim 1, wherein themaximum capacity is set to a value above which the gas furnace is notcapable of operating.
 7. The modulating gas furnace of claim 1, whereinthe maximum capacity is set to 100% of the output capability of the gasfurnace, the minimum capacity is set to 40% of the maximum capacity, andthe intermediate capacity is set to 65% of the maximum capacity.
 8. Ageneral-purpose processor system for a modulating gas furnace, thegeneral-purpose processor system being configured to: monitor adifferential pressure associated with the modulating gas furnace; learnan intermediate value associated with an intermediate capacity between aminimum output capacity of the gas furnace and a maximum output capacityof the gas furnace; learn one of a high value associated with themaximum output capacity and a low value associated with a minimum outputcapacity; establish an estimated operating curve using either theintermediate value and the low value or using the intermediate value andthe high value; and control the gas furnace in accordance with theestimated operating curve.
 9. The general-purpose processor system ofclaim 8, wherein the estimated operating curve is established using theintermediate value and the low value if a demand for heat is equal to orgreater than the minimum capacity and is less than or equal to theintermediate capacity and wherein the estimated operating curve isestablished using the intermediate value and the high value if thedemand for heat is greater than the intermediate capacity.
 10. Thegeneral-purpose processor system of claim 8, the general-purposeprocessor system being further configured to: learn the high value ifthe estimated operating curve is based on the intermediate value and thelow value; learn the low value if the estimated operating curve is basedon the intermediate value and the high value; establish an actualoperating curve based on the low value and the high value; and operatethe modulating gas furnace in accordance with the actual operatingcurve.
 11. The general-purpose processor system of claim 10, wherein atleast one of the low value and the high value and at least one of theestimated operating curve and the actual operating curve are relearnedin response to a relearn command.
 12. The general-purpose processorsystem of claim 8, wherein the minimum capacity is set to a value belowwhich a gas flame of the gas furnace is likely to extinguish.
 13. Thegeneral-purpose processor system of claim 8, wherein the maximumcapacity is set to a value above which the gas furnace is not capable ofoperating.
 14. The general-purpose processor system of claim 8, whereinthe maximum capacity is set to 100% of the output capability of the gasfurnace, the minimum capacity is set to 40% of the maximum capacity, andthe intermediate capacity is set to 65% of the maximum capacity.